Some kernel config symbols we need to enable in some configurations
don't have a "prompt" entry in the Kconfig files, meaning that
make oldconfig (or any other make fooconfig) won't change them.
So we add a Kconfig.local where they can be selected
In this patch we hardcode the required symbol. Next step would be
to generalise so that they could be specified in something like a
config.kernel.hiddenConfig attrset
